Process of curing an acrylic elastomer



United States Patent 3,396,152 PROCESS OF CURING AN ACRYLIC ELASTOMER James C. W. Henning, Akron, Ohio, assignor to The Firestone Tire & Rubber Company, Akron, Ohio, a

corporation of Ohio No Drawing. Filed Mar. 23, 1964, Ser. No. 354,130

2 Claims. (Cl. 260-795) ST ACT F, RE.

An acrylic elastomer, preferably an ester of an alkyl acrylate, methacrylate or ethacrylate in which the alkyl group contains 1 to 8 carbon atoms is milled with an organic peroxide and sulfur on a hot mill and then heated to cure. The elastomer is usually reinforced with a carbon black, a silicon compound, clay, calcium carbonate, etc. The elastomer is intended for use in gaskets, hydraulic hose, electrical insulators, etc.

if the stock'containing them is allowed-t stand for any considerable period before curing. Also, amines tend to corrode the curing mold so that elastomer stock sticks to it, and light goods cured in such molds become colored.

With the peroxide cure of this invention there is no discoloration of the mold, and there is less tendency for the stock to adhere to the mold so that much less mold dope is required. Also, the cure can be carried out within a wider temperature rangefor example, 280 to 400 F. With a higher temperature the cure takes only a very few minutes. The stocks compounded with amine curing agents tend to scorch but the peroxide curing agents of this invention have no tendency to scorch, except for a few such peroxides as benzoyl peroxide. An additional advantage in the use of the peroxides is that they are more permanent than the amines and stocks containing them may be stored for considerable lengths of time before being cured,=and a further -adv'antage in the use of the preferred peroxides is their freedom from any tendency toscorch..

The acrylic elastomers rare. copolymers of an alkyl acrylate or alkylalkacrylate ,with aminor amount of one or more additional monomers including acrylonitrile, styrene, substituted styrenes, or a diene, including. for ex-.

ample: butadiene, isoprene, piperylene, dimethyl buta- 3,396,152 Patented Aug. 6, 1968 sistance of the cured elastomer. The preferred peroxides are:

Dicumyl peroxide, otherwise known as bis(alpha, alpha-dimethyl dicumyl peroxide) (marketed as a mixture of parts of the peroxide and 60 parts of calcium carbonate, by Hercules Powder Co. as Di-Cup 40C).

1,3-bis(alpha-t-butylperoxypropyl) benzene (marketed by Hercules Powder Co. as Hercules 8-890).

2,5-bis(t-butylperoxy)-2,5-dimethyl hexane (marketed with inert carrier by R. T. Vanderbilt Co. as Varox, and by Lucidol Division of Wallace and Tierman as Luperco (101XL).

2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-S (marketed by said Lucidol Division on 50% inert filler as Luperco 130XL).

Other peroxides which may be used include di-t butyl peroxide and benzoyl peroxide. The elastomers may be cured at temperatures of 280 to 400 F., for a time which varies inversely with the temperature, such as 90 to 1.5 minutes for the temperatures given. The temperature used will generally be about 300 to 325 F. for a time within the range of 10 to 30 minutes.

For the cure, about 1 to 10 phr. (used herein to refer to parts per 100' parts of rubber or elastomer), and preferably 2 to 5 phr. of peroxide and 0.1 to 5 phr. of sulfur are used depending upon the peroxide employed.

The acrylic elastomers include a copolymer of 95 percent ethyl acrylate and 5 percent acrylonitrile (known as Acrylon EA-S a copolymer of 88 percent butyl acrylate and 12 percent acrylonitrile (known as Acrylon BA 12),

.. the ethyl acrylate base elastomer known as Thiacril Rubdiene, the pentadienes, the hexadienes, cyclope ntadiene, F

cyclohexadiene, cyclooctadiene, etc. As is known in the art, in such elastomers which include a diene monomer, this monomer constitutes no more than 10 percent of the total weight of the copolymer. The ester may be a lower alkyl acrylate, methacrylate or ethacrylate, or a mixture thereof, 'as is known in the art. The lower alkyl groups include methyl, ethyl, propyl, isopropyl, the butyls, the amyls, the hexyls, the heptyls and the octyls. A small percentage of modifier may be included in the copolymer, as is known in the art.

The peroxide curing agents are organic peroxides well known in the elastomer art. The inclusion of sulfur as a co-curing agent improves the heat resistance and tear re- TABLE I The basic recipe for the runs recorded in this table was:

Acrylon EA-S HAF Black Various ingredients were added to this for each run as indicated below.

Run No 1 2 3 4 5 Stearie acid Sulf Tetrone A. HMDAC. Diak NO. 2" 1 Di-Cup 40C Cure Temperature: 310 F., Stress- Stain Properties at 73 F.:

300% Modulus, p s i 30 minutes 40 minutes" 50 minutes. Tensile Strength, p.s.1.:

30 minutes 40 minutes 50 minutes *TETA=Triethylene tetramine.

*Tetrone A=Dipentarnethylenethiuram tetrasulfide. "*HMDAC =Hexamethylenediamine carbamate. *Diak No. 2=Ethylenediamine earbarnate.

, g H3 Normal stress-strain properties at 73 F. are shown in Table I for amineand peroxide-cure systems. (Runs 1-5 All stocks were mixed in masterbatch form in a Banbury mixer; however, the stock finals had to be handled difierently. Runs containing triethylene tetramine (Runs The examples of '-Table H utilize sulfur as a co-curing agent with several difierent peroxides. The unaged properties of each of these stocks is good and they give excellent properties after aging as shown by their retained tensile strength and elongation after aging 8 hours at 1 and 2) had to be finaled on a cold 111111 with lotsof cool- 400 F. In efiect,the mrtral agrngpenod serves as a posting water to prevent scorching. An initial stock finaled at cure thereby enhancing the properties of the stocks. The 100 F., setup before all of the curing agent could be examples include reinforcing black, and Run no; 10 incorporated into the batch. Although the stocks containincludes a semi-reinforcing black which giv'e good ing other curing agents (Runs 3 and 4) were more stable properties, and excellent aging whenconipa eiwith Run than the stocks containing triethylene tetramine, caution No. 6. a; z i

TABLE III 1 Run No. 11 12 13 14 Acrylon EA-5- -100 100 Thiacril 44 100 Acrylon BA12 100 100 H F mm: 50 e0 e0 50 50 Stearic acid 1.0 1.0 1.0 1.0 1.0 Sulfur 0.5 1.0 1.0 TETA 2.0 2.0 V Tetrone A 2. 0 2. 0

Di-Cup 40C Luperco 130XL 8. 0 Hercules 3-8 2. 0 Cure: Min./ F 60/320 60/310 45/310 /310 30/310 Stress-Strain Properties at 73 F.:

Unaged:

300% Modulus, p.s.i Tensile Strength, p.s.i Ultimate Elongation Aged 8 Hrs. at F Tensile Strength, p Ultimate Elongation, p

Stress-Strain Properties at 400 F.:

Unaged' Tensile Strength, p.s.i 440 310 270 Ultimate Elongation, percent-- 330 450 360 200 100 Aged 8 Hits. at F 400 400 400 400 350 Tensile Strength, p.s.i 310 80 430 420 390 Ultimate Elongation, percent 160 90 160 70 60 had to be exercised to prevent scorching. On the other hand, the stock containing the dicumyl peroxide and sulfur (stock No. 5) was finaled on a hot mill at 170 F. and processed quite satisfactorily, and produced a good balance in tensile strength and ultimate elongation. Stocks of this nature could be finaled in a Banbury mixer using temperatures not exceeding about 225 F. using the preferred peroxides of this invention which do not become active until heated to about 300 F. The peroxides which are active at lower temperatures, such as di-tertiary butyl peroxide and 'benzoyl peroxide do not, of course, show this processing ease and safety, but lie within the scope of the invention.

TABLE IV TABLE H The basic recipe for the runs recorded in this table was: The basic recipe for the runs recorded in this table was: Acrylon EA-S 100 Arcylon 5 10o Stearic acid 1.0 HAF Black 40 Sulfur 1.0 Stearic acid 1.0

Run N 6 7 8 9 10 HAF Black 60 60 60 FEF Black 60 Di-Cup 40C 11 11 Hercules S-89 2 Luperco l01-XL. 8 Luperco 130- 8 cl llre i 60 min. at 310 F., Stress-Strain Properties at 7 v Y Unaged:

300% Modulu, p.S.L 825 900 875 1, 850 675 Tensile Strength, p.s.i 1, 425 1, 625 1, 525 2, 075 1, 175 Ultimate Elongation, percent 570 550 600 350 620 Aged 8 hrs. at 400 F.:

Percent Retained Tensile 126 111 121 Percent Retained Elongation 58 65 52 74 53 Stress-strain properties at 400 F.:

Unaged:

Tensile Strength, p.s.i 350 350 330 540 270 Ultimate Elongation, Percent. 350 420 370 330 Aged 8 Hrs. at 400 F.:

Percent Retained Tensile 117 120 119 106 137 Percent Retained Elongation 40 40 43 69 42 Unaged:

300% Modulus, p.s.i 475 1, 300 225 225 700 Tensile Strength, p.s.i 1, 675 1, 650 1, 350 l, 400 1, 275 1, 800 Ultimate Elongation, percent 700 370 260 870 940 600 Aged 8 Hrs. at 400 F;

300% Modulus, p.s.i 1,175 1, 250 600 675 1,050 Tensile Strength, p.s.l 1,850 1,300 1,075 1,500 1,575 1,700 Ultimate Elongation, percent 410 310 230 640 560 450 Hot Ring Tear, Cured 45 min. at 310 F.,

lbs/in. at 212 F 191 as 14 147 136 131 Shfiore A Hardness, Cured 50 min. at 310 Instant a 42 42 as 32 36 fi-second 31 40 40 29 27 34 TABLE v 15 tensile strength, higher elongation, better aging resistance and much better tear resistance. An additional outstanding The baslc Tempe for runs recorded In thlS table Was! characteristic of the peroxide-sulfur cure is shown by a Thiacrn 44 100 comparison of the products of Run with Run 5 in Stearic acid 1.0 Table III. These stocks have the same level of peroxide Sulfur L0 20 and sulfur and show the high resistance to overcuring Lupgrco 130 XL characteristic of this system; viz 60 minutes at 320 F. used for curing in Run 5 compared to 30 minutes at 310 Run No 21 22 23 24 25 26 F. in Run 20. The modulus difference is no greater than HAF Bl k 30 expected for the 10 phr. difference in the black loadings. inc B13251; w l 25 What I claim IS: g' g g h 1. The process of curing an elastomer of the class con- Silene EF Triethanolamine Cure Temperature:

310 F. for times indicated, StgeBs s-Strain Properties at 300% Modulus, p s i 30mi11utes. 150 625 350 200 375 500 60 nn'putes 275 850 475 300 325 550 90minutes 300 875 625 325 325 575 Tensile Strength, p.s.i.:

30mi11utes 850 800 1,600 725 1,100 775 60minutes .1,575 1,075 1,875 $251,150 800 90 minutes 1,800 1,100 2,075 9001,000 800 Ultimate Elongation,

Percent:

30minutes 690 670 800 900 950 690 60 minutes" 570 690 750 940 570 90 minutes 810 550 690 780 930 610 The results show good results with different reinforcing agents used in different amounts. These reinforcing agents were:

HAP Black is a furnace black EPC Black is a channel black MT Black is a thermal black Hi-Sil 233 is a hydrated silica Silene EF is a calcium silicate It is customary to add a small amount of amine with hydrated silicas, and in this instance triethanolamine was used.

Difierent amounts of sulfur and peroxide were used in the experiments of Table IV to find optimum peroxide and sulfur levels. Run 17 shows that peroxide alone gives a good tight cure, but the tear resistance is low. Run 20 which used the same peroxide level, but with 0.5 phr. of added sulfur shows the following beneficial effects: higher sisting of copolymers of alkyl acrylates, alkyl methacrylates, and alkyl ethacrylates in which the alkyl group contains 1-8 carbon atoms which process comprises hot mixing 0.1 to 5 parts of sulfur and 1 to 10 parts of an organic curing agent into parts of the elastomer at a temperature in excess of 100 F. and not exceeding 225 F., the peroxide being from the class consisting of bis(alpha, alpha-methyl dicumyl peroxide), 1,3-bis(alpha-t-butylperoxylpropyl) benzene, 2,5 bis(t butylperoxy) -2,5-dimethyl hexane and 2,5 dimethyl-2,5-di(t-butylperoxy) hexyne-3, and then heating at 280 to 400 F. until the elastomer is cured.

2. The process of claim 1 in which the peroxide is his (alpha, alpha-methyl dicumyl peroxide), the amount used is substantially 4 parts per 100 parts of the elastomer with substantially 0.5 to 1 part of sulfur, and the elastomer is from the class consisting of alkyl esters of acrylates, methacrylates and ethacrylates in which the alkyl group contains 1 to 8 carbon atoms.

References Cited UNITED STATES PATENTS 2,925,406 2/1960 McCurdy et al 26079.5 3,012,016 12/1961 Kirk et a1 260'79.5 3,127,379 3/ 1964 Natta et a1 260-79.5

FOREIGN PATENTS 898,264 6/ 1962 Great Britain.

ALLAN LIEBERMAN, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 ,396 ,152 August 6 1968 James C. W. Henning It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 11, Tierman" should read Tiernan same column, in the table, second column, line 4 thereof, cancel H2 OH.

Signed and sealed this 3rd day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, J r.

Attesting Officer 

